Viral spread from the mucosal sites of infection to target tissues and organs is a fundamental step in the pathogenesis of many viral diseases. Viruses typically use the bloodstream and lymphatic system as the pathway for delivering virus to organs and tissues throughout the body. Although general principles of lymphatic-hematogenous viral spread are understood, little is known about the viral and cellular determinants that govern virus dissemination by these routes. The overall objective of experiments described in this application is to define mechanisms and functional consequences of viral spread via the blood. Work proposed in this application seeks determine how mammalian orthoreovirus (reovirus) traffics from an initial site of inoculation to the bloodstream. Previous studies revealed that nonstructural protein s1s is required for reovirus spread by hematogenous routes. New preliminary data indicates that s1s functions to antagonize type 1-interferon (IFN) responses to enable efficient reovirus replication. Together, these data indicate that s1s-mediated inactivation of host IFN responses allows reovirus to overcome cellular barriers that restrict access to the bloodstream. Based on preliminary studies, experiments in Specific Aim 1 will define cell types required to disseminate reovirus from the site of inoculation to the bloodstream. Experiments in Specific Aim 2 will define the mechanism by which reovirus nonstructural protein s1s antagonizes the host IFN response. Experiments proposed in Specific Aim 3 will examine how reovirus bloodstream spread contributes to viral clearance from the central nervous system. These experiments will use a combination of powerful mouse and reovirus genetics systems to address how reovirus gains access to the blood. This research will have broad general impact in uncovering unifying principles that govern systemic viral spread.